Vertical-cavity surface-emitting lasers (VCSELs) are attractive devices for high-power applications. Since the output power and efficiency of VCSELs in CW-operation is usually limited by thermal effects, bottom-emitting VCSELs with an emission wavelength of around 980 nm are preferred for high-power applications. They are mounted p-side down in order to have the active area close to the heat sink and to avoid high thermal resistances. Nevertheless, bottom-emitting VCSELs required quite some distributed Bragg reflectors (DBRs) on the p-side of the VCSEL in order to achieve moderate threshold currents and to avoid outcoupling losses on the wrong facet. Since the number of p-DBRs determines the thermal performance, the design of bottom-emitting VCSELs for high-power applications usually needs a difficult compromise between optical performance and thermal management.
State-of-the-art vertical-cavity surface-emitting lasers that emit to the bottom (substrate) side require approximately 30 pairs of distributed Bragg reflectors (DBRs) on the p-side of the laser diode. If these devices are intended to be used in a high-power application, they are typically mounted p-side down and the number of p-DBRs defines the thermal resistance.
In the publication “Hybrid dielectric/metal reflector for low threshold vertical-cavity surface emitting lasers”, M. R. Daniel et al, Electron. Lett. 1997 Vol. 33 No. 20, pp 1704 ff, a bottom emitting VCSEL is disclosed comprising a dielectric top mirror with a metallic overcoat to reduce the mirror thickness. However, coming along with the reduced mirror thickness, not only the bandwidth increases, as disclosed in the above mentioned publication, but also the amplification of a variety of modes. This on the other hand is detrimental for the beam quality.